# std::scoped_lock

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Defined in header `<mutex>` |  |
template< class... MutexTypes > class scoped_lock; |  | (since C++17)

The class `scoped_lock` is a mutex wrapper that provides a convenient RAII-style mechanism for owning zero or more mutexes for the duration of a scoped block.

When a `scoped_lock` object is created, it attempts to take ownership of the mutexes it is given. When control leaves the scope in which the `scoped_lock` object was created, the `scoped_lock` is destructed and the mutexes are released. If several mutexes are given, deadlock avoidance algorithm is used as if by std::lock.

The `scoped_lock` class is non-copyable.

### Template parameters

MutexTypes | - | the types of the mutexes to lock. The types must meet the Lockable requirements unless sizeof...(MutexTypes) == 1, in which case the only type must meet BasicLockable

### Member types

Member type | Definition
`mutex_type` (conditionally present) | If sizeof...(MutexTypes) == 1, member type `mutex_type` is the same as `Mutex`, the sole type in `MutexTypes...`. Otherwise, there is no member `mutex_type`.

### Member functions

(constructor) | constructs a `scoped_lock`, optionally locking the given mutexes (public member function)
(destructor) | destructs the `scoped_lock` object, unlocks the underlying mutexes (public member function)
operator= [deleted] | not copy-assignable (public member function)

### Notes

A common beginner error is to "forget" to give a `scoped_lock` variable a name, e.g. std::scoped_lock(mtx); (which default constructs a `scoped_lock` variable named `mtx`) or std::scoped_lock{mtx}; (which constructs a prvalue object that is immediately destroyed), thereby not actually constructing a lock that holds a mutex for the rest of the scope.

Feature-test macro | Value | Std | Feature
--- | --- | --- | ---
`__cpp_lib_scoped_lock` | `201703L` | (C++17) | `std::scoped_lock`

### Example

The following example uses `std::scoped_lock` to lock pairs of mutexes without deadlock and is RAII-style.

Run this code

    #include <chrono>
    #include <functional>
    #include <iostream>
    #include <mutex>
    #include <string>
    #include <thread>
    #include <vector>
    using namespace std::chrono_literals;

    struct Employee
    {
        std::vector<std::string> lunch_partners;
        std::string id;
        std::mutex m;
        Employee(std::string id) : id(id) {}
        std::string partners() const
        {
            std::string ret = "Employee " + id + " has lunch partners: ";
            for (int count{}; const auto& partner : lunch_partners)
                ret += (count++ ? ", " : "") + partner;
            return ret;
        }
    };

    void send_mail(Employee&, Employee&)
    {
        // Simulate a time-consuming messaging operation
        std::this_thread::sleep_for(1s);
    }

    void assign_lunch_partner(Employee& e1, Employee& e2)
    {
        static std::mutex io_mutex;
        {
            std::lock_guard<std::mutex> lk(io_mutex);
            std::cout << e1.id << " and " << e2.id << " are waiting for locks" << std::endl;
        }

        {
            // Use std::scoped_lock to acquire two locks without worrying about
            // other calls to assign_lunch_partner deadlocking us
            // and it also provides a convenient RAII-style mechanism

            std::scoped_lock lock(e1.m, e2.m);

            // Equivalent code 1 (using std::lock and std::lock_guard)
            // std::lock(e1.m, e2.m);
            // std::lock_guard<std::mutex> lk1(e1.m, std::adopt_lock);
            // std::lock_guard<std::mutex> lk2(e2.m, std::adopt_lock);

            // Equivalent code 2 (if unique_locks are needed, e.g. for condition variables)
            // std::unique_lock<std::mutex> lk1(e1.m, std::defer_lock);
            // std::unique_lock<std::mutex> lk2(e2.m, std::defer_lock);
            // std::lock(lk1, lk2);
            {
                std::lock_guard<std::mutex> lk(io_mutex);
                std::cout << e1.id << " and " << e2.id << " got locks" << std::endl;
            }
            e1.lunch_partners.push_back(e2.id);
            e2.lunch_partners.push_back(e1.id);
        }

        send_mail(e1, e2);
        send_mail(e2, e1);
    }

    int main()
    {
        Employee alice("Alice"), bob("Bob"), christina("Christina"), dave("Dave");

        // Assign in parallel threads because mailing users about lunch assignments
        // takes a long time
        std::vector<std::thread> threads;
        threads.emplace_back(assign_lunch_partner, std::ref(alice), std::ref(bob));
        threads.emplace_back(assign_lunch_partner, std::ref(christina), std::ref(bob));
        threads.emplace_back(assign_lunch_partner, std::ref(christina), std::ref(alice));
        threads.emplace_back(assign_lunch_partner, std::ref(dave), std::ref(bob));

        for (auto& thread : threads)
            thread.join();
        std::cout << alice.partners() << '\n'  << bob.partners() << '\n'
                  << christina.partners() << '\n' << dave.partners() << '\n';
    }

Possible output:

    Alice and Bob are waiting for locks
    Alice and Bob got locks
    Christina and Bob are waiting for locks
    Christina and Alice are waiting for locks
    Dave and Bob are waiting for locks
    Dave and Bob got locks
    Christina and Alice got locks
    Christina and Bob got locks
    Employee Alice has lunch partners: Bob, Christina
    Employee Bob has lunch partners: Alice, Dave, Christina
    Employee Christina has lunch partners: Alice, Bob
    Employee Dave has lunch partners: Bob

### Defect reports

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

DR | Applied to | Behavior as published | Correct behavior
--- | --- | --- | ---
LWG 2981 | C++17 | redundant deduction guide from `scoped_lock<MutexTypes...>` was provided | removed

### See also

unique_lock (C++11) | implements movable mutex ownership wrapper (class template)
lock_guard (C++11) | implements a strictly scope-based mutex ownership wrapper (class template)

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